Optimising Cardiac Diffusion Tensor Imaging In Vivo: More Directions or Repetitions?

Abstract

Background: Cardiac diffusion tensor imaging (cDTI) is sensitive to imaging parameters including the number of unique diffusion encoding directions (ND) and number of repetitions (NR; analogous to number of signal averages or NSA). However, there is no clear guidance for optimising these parameters in the clinical setting.

Methods: Spin echo cDTI data with 2^nd order motion compensated diffusion encoding gradients were acquired in ten healthy volunteers on a 3T MRI scanner with different diffusion encoding schemes in pseudo-randomised order. The data were subsampled to yield 96 acquisition schemes with 6 ≤ ND ≤ 30 and 33 ≤ total number of acquisitions (NA_all) ≤ 180. Stratified bootstrapping with robust fitting was performed to assess the accuracy and precision of each acquisition scheme. This was quantified across a mid-ventricular short-axis slice in terms of root mean squared difference (RMSD) with respect to the full reference dataset, and standard deviation (SD) across bootstrap samples respectively.

Results: For the same acquisition time, the ND = 30 schemes had on average 48%, 40%, 34% and 34% lower RMSD and 6.2%, 7.4%, 10% and 5.6% lower SD in MD, FA, HA and |E2A| compared to the ND = 6 schemes. Given a fixed number of high b-value acquisitions, there was a trend towards lower RMSD and SD of MD and FA with increasing numbers of low b-value acquisitions. Higher NA_all with longer acquisition times led to improved accuracy in all metrics whereby quadrupling NA_all from 40 to 160 volumes led to a 20%, 39%, 11% and 5.4% reduction in RMSD of MD, FA, HA and |E2A| respectively, averaged across six diffusion encoding schemes. Precision was also improved with a corresponding 53%, 50%, 53% and 36% reduction in SD.

Conclusions: We observed that accuracy and precision were enhanced by (i) prioritising number of diffusion encoding directions over number of repetitions given a fixed acquisition time, (ii) acquiring sufficient low b-value data, (iii) using longer protocols where feasible. For clinically relevant protocols, our findings support the use of ND = 30 and NA_b50:NA_b500 ≥ 1/3 for better accuracy and precision in cDTI parameters. These findings are intended to help guide protocol optimisation for harmonisation of cDTI.